The html-helper-mode adds an HTML menu to Emacs, where you can use shortcuts for almost any HTML tags/entities/elements.

Reloading an Emacs feature
Let's say you hacked the html-helper-mode.el file to suit your needs. Changes in the files do not reflect the behaviours of html-helper-mode immediately - you have to reload the package first:

M-x unload-feature
RET
html-helper-mode
RET

If you have added the line (autoload 'html-helper-mode ... (see above) to your .emacs file, it is sufficient to open a new file with the .html to reload html-helper-mode.
You can also reload the feature manually:

M-x load-library
RET
html-helper-mode
RET

(Remember that in the case of html-helper-mode, the package tempo.el is also loaded, so if you want to disable all html-helper-mode features manually, you have to unload-feauture tempo as well.)
Of course, unloading/loading a feature isn't limited to html-helper-mode, but can be applied to any Emacs feature. You can get a list of loaded features:

M-x unload-feature
RET
(press TAB here to see a list of loaded features)

replace-regexp (or query-replace-regexp)

Search and replace:

M-x query-replace-regexp

Replace 4 or more digits at the beginning of a line with the sign '>':

M-x replace-regexp RET
^[0-9]\{4,\} RET
> RET

Search and replace 6 digits with an empty string:

M-x query-replace-regexp
[0-9]\{6\}
RET
RET

Search and replace 6 or more digits followed by a space with an empty string:

M-x query-replace-regexp
[0-9]\{5,\}[ ]
RET
RET

Search and replace 3 or more digits preceded by possible space(es) and followed by a space, with one space:

M-x query-replace-regexp
[ ]*[0-9]\{3,\}[ ]
RET
RET

Search the first occurence of a dot in a line (.) and remove the rest of the line, including the dot:

M-x query-replace-regexp
[.].*$
RET
RET

The same as above, but with a colon instead as a dot:
(This is useful when you have a file with a grep results from various files, and just want a list of the files; use sort -u from the command line to sort the list and eliminate duplicates.)

M-x query-replace-regexp
:.*$
RET
RET

Insert a TAB at the beginning of a line, and add a continuation character at the end:
(This is useful when for example you have a list of files, and want to list them in a Makefile or a shell script.)

Rectangles in Emacs
You can cut and paste rectangles, i.e. columns, in emacs.
Put the cursor at the first row and column of the rectangle you want to cut or copy.
Select the rectangle (C-SPC) and move the cursor to the last row and column you want to include.
To cut (kill) a rectangle:

C-x r k

To paste (yank) a rectangle (starting where the cursor is positioned):

C-x C-b (List all buffers - splits window into two buffers)
C-x o (Switch to buffer list)
d (shows a D at the beginning of each line with the name of the buffer to be deleted)
x (kill all selected buffers, i.e. beginning with a D)
C-x k (kill the buffer list itself)
C-x 1 (unsplit window)

The -f iso-accents-mode arguments are not necessary, but convienient when typing in other languages than English.

Specify editor: (newer versions of Emacs, details explained below)

emacs %f

Emacs and mixing human languages in the same document:
In Emacs 22, iso-accents-mode is deprecated.
One (in my opinion not very easy-to-use) solution is to use C-\ latin-1-prefix instead (inside Emacs, not as a command line option).
If you plan to type in different languages, and even mix languages in the same document, including both Emacs and other applications, a better solution is to configure X (assuming you are using a UNIX-like graphical environment) to handle non-English character.
See Configure X to handle non-English characters for details.